1. Field of the Invention
The present invention relates to a production apparatus for producing anisotropic rare earth magnet powder by applying hydrogen heat treatment which induces hydrogenation and hydrogen desorption in rare earth magnet material.
2. Description of the Related Art
Recently the rare earth anisotropic magnet powders are increasingly used in bonded magnets, mainly due to their good magnetic properties such as high maximum energy product ((BH) max ), high residual magnetic flux density (Br) and high intrinsic coersivity (iHc). Among various producing methods of the rare earth magnet powders with good magnetic properties, it is known that the hydrogen heat treatment is quite effective. The hydrogen heat treatment consists of the hydrogenation process in which the raw magnet powder is kept at 750.degree. C.-950.degree. C. in hydrogen atmosphere to make the powder absorb hydrogen and subsequent hydrogen desorption process in which the powder is compelled to release the hydrogen in vacuum. By applying hydrogen heat treatment, iHc is enhanced because of refinement of the grain, and Br is improved by an alignment of the grain orientation in the material.
The above mentioned hydrogen heat treatment, however, has a drawback that the method is not suitable to mass production. It is because the required temperature control of the material is too severe to attain by the conventional technology in a large scale production.
In the hydrogen heat treatment, the small deviation of treatment temperature from the desired value either in the hydrogenation process or in the hydrogen desorption process causes significant deterioration of the magnetic properties of the obtained magnet powder. Therefore it is required that the temperature is controlled precisely in the whole material in both processes. However, there is a formidable problem to keep the temperature constant. The reaction between the magnet powder and hydrogen is a self exciting reaction, the hydrogenation being exothermic reaction and the hydrogen desorption being endothermic reaction. The heat generated in the reactions is proportional to the material mass. The heat tends to make the treatment temperature deviate from a desired range. So it is difficult to keep the temperature in a desired range in mass production.
We solved this problem by inventing a production method and an apparatus for hydrogen heat treatment that is disclosed in Japanese Patent Application Laid-open (Kokai) No.9-251912. The invented apparatus is characterized by sets of a processing vessel and a heat compensating vessel in contact. The heat generated by the exothermic reaction during hydrogenation process or by the exothermic reaction during hydrogen desorption process is compensated by the counter reaction of heat generating material contained in the heat compensating vessel. As a result, the treatment temperature of the hydrogen heat treatment can be easily controlled within a desired treatment temperature range. The controllability of the method is independent of the production scale so that mass production by the hydrogen heat treatment can be set into practice.
However the invented apparatus had three major drawbacks because it utilized a batch-type furnace. The first drawback is poor time efficiency for the treatment. The second is the oxidization of the powder. The third is inhomogeneity in the processed powder.
The poor time efficiency mainly comes from the following two reasons. One is the need of heating and cooling time of the furnace. The heating from room temperature to the treatment temperature and the cooling from treatment temperature to the room temperature takes certain time for each processing batch. The second is the handling time for feeding the powder material in the apparatus and taking them out. The handling requires special care. When feeding the raw material into the furnace, the powder must be distributed well in the processing vessel to assure the sufficient contact area with the heat compensating vessel for the precise temperature control. The effective feeding method in the interlaced structure of processing vessel and heat compensating vessel was not established in the above invention, so the feeding requires long time. When taking the processed material out of the furnace, the powder is usually aggregated to lumps and need to be crushed before taking them out. It also requires long time.
The second drawback is that the material may be oxidized during feeding of the raw magnet powder or taking out the processed magnet powder because processing room of the furnace is exposed to the ambient atmosphere.
The third drawback, the inhomogeneity of the processed powder, is brought about during the heating and cooling steps of the furnace. The heating and cooling of the material is done by heat transfer between the processing vessel and the material. Because the degree of the heat transfer is different in the position in the processing vessel, the temperature inhomogeneity takes place in the material. This temperature inhomogeneity brings about the magnetic property inhomogeneity.